1. Field of the Invention:
The present invention relates to an Al alloy sheet metal excellent in formability which is used for the formation of fins of a heat exchanger by forming penetrating holes for the tubes of a heat exchanger by piercing, burling, ironing, and flanging, and to a process for the preparation of such Al alloy metal sheets.
2. Description of the Prior Art:
As the conventional method for forming fins of a heat exchanger of the tube and fin type, namely a method for forming tube-penetrating holes, there can be mentioned the so-called draw-flanging method including the steps of piercing, burling and flaring as shown in FIGS. 1a-1c and a Burr oak method (Weldun method) including at least one drawing (overhanging) step, piercing, burling and flaring as shown in FIGS. 2a-2f. The Al alloy metal sheets which have generally been used for these methods are those of the pure Al series represented by A1050 (JIS Standard) and of so-called mild materials having a tensile strength .alpha.b of 7 to 13 Kg/mm.sup.2, such as O temper material or H.sub.22 temper material.
Recently, in order to lower the manufacturing cost, it has been desired to reduce the thickness of these fin-forming materials. However, when these soft temper materials which have heretofore been used are reduced in thickness, various problems and difficulties are brought about in connection with the forming technique and the use of the resulting fins, and they cannot be put into practical use. More specifically, handling of the same involves difficulties and defects such as cracks are readily formed during the forming step. Further, a sufficient adhesion cannot be obtained between the fins and tubes and the heat transfer efficiency is reduced.
Use of a hard material having a tensile strength .alpha.b of about 18 Kg/mm.sup.2 has been proposed as a fin-forming material for overcoming the foregoing disadvantages, and as the fin-forming method using such hard material, there has been proposed a method comprising the steps of piercing, burling, ironing and flanging as shown in FIGS. 3a-3d.
However, when conventional Al materials are hardened and used for this fin-forming method, fine cracks are formed in the collar end portion after the ironing step and large cracks are formed during the subsequent flanging step. Accordingly, development of Al alloy metal sheets having a high strength and being excellent in formability, which can be conveniently used for the formation of fins of a heat exchanger, has been desired in the art.